I think the question is really more about rerouting the signal from iTunes than anything specific to Amadeus Pro. Of course, iTunes is designed to resist digital "porting" in real time. Soundflower and Audio Hijack are often used to strip and reroute audio signals.

Also, if you could manage to get the signal path right, and are using one computer to do everything, you are going to be creating pretty good sized files on the machine which is both recording and playing. You might want to consider other options, such as an audio mix through an A/D converter, using an iPod or other decidated playback device.

There may also be some legal implications in the use or rebroadcast of copyrighted material.

I'm a community radio DJ and I'm wanting to pre-record some shows.
Amadeus pro plays my tracks from iTunes, I want to use the Macs microphone to add spoken intros to songs I'm playing through Amadeus and record it all as I go.